Except loads of M3 MT drivers on here can't even manage a shift into second without crunching the gears - imagine what damage they could do on a dragstrip.
But lets say .25 of a sec for an MT shift...given that the DCT S6 powershift is just a few ms an MT car is still losing 0.25 sec per shift.
You'll need a lot of extra HP to pull back .5 sec over 2 shifts

Nope, DCT doesn't shift instantly. It takes about 0.15 seconds (150 milliseconds) in S6, so it will lose (gain) about 0.1 at most per shift against a real 6MT driver.